The flows of fluids running in channels are classified into two kinds. One is a single-phase flow in which a gas or a liquid flows in a single phase, and the other is a gas-liquid two-phase flow in which a gas and a liquid are mixed to flow.
As a device including the single-phase flow, a heat exchanger for heating or cooling is exemplified. Improvements for reducing the thermal resistance between a heat source and a fluid have been continued.
In recent years, with increasing calorific values and sizes of semiconductor power modules which are incorporated into power converters in electric vehicles, hybrid cars, large wind power generators, railway vehicles or the like, a cooling device as a heat exchanger having a higher cooling power has been required.
As regards that, an invention related to a form of a radiation fin for directly cooling with a liquid current (see Patent Document 1), an invention wherein a cooling medium circulating between standing walls of a comb teeth member is caused to circle by a plurality of guides so as to make an uneven temperature distribution of the cooling medium difficult to occur in the cooling medium channel (see Patent Document 2), or the like has been proposed.
As devices including the gas-liquid two-phase flow, an evaporator, an absorber, a regenerator and a condenser of a lithium bromide system (LiBr) absorption refrigerating machine are exemplified.
The lithium bromide system absorption refrigerating machine comprises an evaporator, an absorber, a regenerator and a condenser. Among these four heat exchangers, three heat exchangers except the regenerator are of a shell & tube type wherein heat exchanger tubes are horizontally disposed within a shell. Water is allowed to flow in the heat exchanger tubes which function as heating tubes or cooling tubes. By devising how to arrange a group of heat exchanger tubes, how many heat exchanger tubes are used and the like, the powers thereof are enhanced.
In the absorber, a liquid film of a lithium bromide solution flowing down on the surface of a cooling tube absorbs steam vaporized in the evaporator, and in the system wherein the absorber and the evaporator communicate with each other, a high vacuum is maintained. When the lithium bromide solution absorbs steam, heat of absorption is generated, and therefore, by cooling water running in the cooling tube, the lithium bromide solution is allowed to cool.
In the condenser, steam generated in the regenerator is turned back into water (a liquid) by condensation on the surface of the cooling tube, which is sent to the evaporator.
As another device including the gas-liquid two-phase flow, a distillation apparatus is exemplified.
The distillation apparatus is aimed at separating a low-boiling component and a high-boiling component. Usually a distillation column is installed on a reboiler, and a condenser is arranged near the column top.
Steam generated by heating in the reboiler moves from the column bottom in the column up to the condenser, in which it is cooled, resulting in a condensate. Part of the condensate is distilled, while the rest thereof is refluxed to the column top. The reflux liquid flowing down and the steam moving upward make countercurrent contact. Since the steam moving upward has large enthalpy, it gives heat to the reflux liquid flowing down so as to vaporize part of the reflux liquid.
At that time, since steam richer in low-boiling component compared with the composition of the reflux liquid is generated, a rectification effect is caused. Since the steam gives heat to the reflux liquid, the temperature abruptly decreases in the neighborhood of the contact surface with the reflux liquid and part of the steam in the vicinity thereof is condensed. Since the condensed part thereof is richer in high-boiling component compared with the main part of the steam, another rectification effect is caused. By accumulating such rectification effects in multiple stages in the direction of the column axis, the low-boiling component and the high-boiling component are separated. As a result, the low-boiling component is condensed in the column top portion, while the high-boiling component is condensed in the column bottom portion (the source of the above explanation of the principle of a distillation apparatus: a thesis for a doctorate in Graduate School of Engineering, Nagoya University, “The development of a technique for analyzing separation performance of a packed water distillation column for hydrogen isotope separation”, Takahiko Sugiyama).
As for the distillation apparatus, a plate column and a packed column, wherein the gas-liquid contact area is enlarged and the turbulence of each phase of the gas-liquid countercurrent is increased so as to enhance the multistage rectification effects have been developed. Even a distillation column with a height of 50 m has been in the actual use.
Both a channel forming body for mounting semiconductor power modules described in the Patent Document 1 and a heat sink for power modules described in the Patent Document 2, have a heat-transfer portion having a very complicated construction in order to increase the heat-transfer coefficient. Consequently, there is a problem that extremely high techniques and a high cost are required for manufacturing them.
As heat exchanger tubes of the above shell & tube type heat exchanger in the lithium bromide system absorption refrigerating machine, high-performance and expensive heat exchanger tubes having improved forms of the inside and outside are used. However, these heat exchanger tubes usually have an overall heat-transfer coefficient of 5000 W/m2·K (tubeside area basis) or less. Therefore, there is a problem that, in order to secure a sufficient heat-transfer area, a large number of heat exchanger tubes should be effectively arranged within the shell, leading to a high cost.
In the absorber, within the liquid film of the lithium bromide solution flowing down in the heat exchanger tube, a temperature boundary layer and a density boundary layer are formed. Therefore, there is a problem that the absorption and diffusion of cooling medium steam into the liquid film of the lithium bromide solution are restricted.
In the condenser, when the condensate flows down in the form of a liquid film on the surface of the heat exchanger tube, heat transfer occurs between the surface of the liquid film and steam through convective heat transfer, while heat transfer occurs between an inner part of the liquid film and the heat-transfer surface through thermal conduction, and the condensation heat transfer coefficient depends on the thickness of the liquid film. There is a problem that the thicker the liquid film becomes, the smaller the condensation heat transfer coefficient becomes.
In the distillation column, when both fluids, a reflux liquid and steam are allowed to slowly flow in the gas-liquid countercurrent contact thereof, vaporization/condensation is sufficiently repeated, resulting in a better mass-transfer efficiency, a higher distillation effect and a smaller pressure loss. As a result, the height of the column can be reduced. However, as for the plate distillation column industrially used in actual, there are problems that the pressure loss is large, and that the height thereof is large since a construction wherein maintenance can be performed is required. On the other hand, as for the packed distillation column, there are problems that drift currents easily occur in a reflux liquid, and that it is also difficult to control the falling rate thereof.